(a) Field of the Invention
The invention relates to an electromagnetic interference (EMI) suppression device and, more particularly, to an EMI suppression device used for the prevention of both common mode and differential mode noises.
(b) Description of the Related Art
Generally, noises that affect signal lines or power lines are categorized as common mode and differential mode noises. Common mode noises, also called asymmetrical-mode noises, are noises in two signal lines having the same phase but a minute difference in amplitude; differential mode noises, also called symmetrical-mode noises, are noises in two signal lines having the same amplitude but with 180 degree phase difference. These two types of noises commonly exist in input or output lines.
Solution elements regularly used for reducing differential mode noises are often made of ferrite beads, which can provide a proper impedance characteristic at high frequencies to absorb unwanted noises. FIGS. 1A and 1B show schematic views where a ferrite bead 106 is adopted to suppress differential mode noises. Referring to FIG. 1A, there is shown a simple RC circuit including a power supply 100, an amplifier 102, a capacitor C and a resistor R. An oscilloscope 104 coupled to RC circuit displays the output waveform disrupted by differential mode noises, as illustrated in FIG. 1A. When a ferrite bead 106 is connected to the RC circuit, shown in FIG. 1B, the impedance characteristic of the ferrite bead 106 functions to absorb the noises, and the RC circuit can finally output a filtered waveform as illustrated in FIG. 1B.
In addition, solution elements regularly used for reducing common mode noises are often made of a common mode choke for removing noises produced from the signal line and the fluctuations of the ground voltage. The operating principle for the common mode choke is that when currents having the same phase (that is, common mode currents) flow in, an impedance for filtering the noise is generated because of the added magnetic induction induced by the currents. To the contrary, when currents in opposite phase (that is, differential mode currents) flow in, the magnetic induction induced by the currents is cancelled by each other. In other words, common mode chokes leave differential mode currents unaffected while selectively attenuates the noise with respect to common mode currents. FIG. 2 shows the noise attenuation at different frequencies done by a common mode choke. It is observed that the filtering effect of a common mode choke is more apparent at high frequencies.
In a prior method, the aforesaid EMI suppression elements for common mode and differential mode noises, namely common mode chokes and ferrite beads, are arbitrarily spread on a printed circuit board (PCB) for reducing both common mode and differential mode noises. However, it is necessary that spaces for various elements are reserved on distinct areas of the PCB, and therefore the space flexibility for a circuit layout is limited and more fabrication time is needed. Furthermore, the method of arbitrarily spreading the various EMI suppression elements over the PCB is liable to raise other unpredictable issues, caused by the wires in the layout having short distances in between.